Low temperature polyimide adhesive compositions and methods relating thereto

ABSTRACT

The present invention relates to a polyimide adhesive composition having a polyimide derived from an aromatic dianhydride and a diamine component, where the diamine component is preferably about 50 to 90 mole % of an aliphatic diamine and about 10 to 50 mole % of an aromatic diamine. In one embodiment, the aliphatic diamine has the structural formula H 2 N—R—NH 2  wherein R is hydrocarbon from C 4  to C 16  and the polyimide adhesive has a glass transition temperature in the range of from 150° C. to 200° C. The present invention also relates to compositions comprising the polyimide adhesive of the present invention, including polyimide metal-clad laminate useful as flexible circuit when metal traces are formed out of the metal used in flexible, rigid, or flex-rigid circuit applications.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to low temperature (less thanabout 225° C.) polyimide based adhesive compositions suitable forelectronics applications, such as bonding films to form: multilayerflexible circuits, rigid-flex circuits, chip scale packaging or thelike. The compositions of the present invention comprise analiphatic-aromatic polyimide component, having advantageous thermalresistance, z-axis coefficient of thermal expansion (CTE) and modulusproperties.

2. Discussion of Related Art

U.S. Pat. No. 5,922,167 to Rosenfeld is directed to polyimide adhesivecompositions, having bonding temperatures taught to be in a range ofabout 250° C. to 450° C.

The present invention relates to prior research disclosed in U.S. Pat.No. 5,298,331, to Kanakarajan, et al. for polyimide adhesives withbonding temperatures in a range of about 250° C. to 275° C. The methodsof manufacture and use described in the Kanakarajan patent are alsoapplicable to the polyimide adhesives of the present invention, andtherefore the Kanakarajan et al. patent is hereby incorporated byreference into this specification for all teachings therein.

SUMMARY OF THE INVENTION

The present invention is directed to an adhesive composition, comprisinga low glass transition temperature (“Tg”) polyimide base polymer. “Basepolymer” as used herein is intended to mean the dominant polymercomponent (at least 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100 weightpercent of all polymers present in the adhesive compositions of thepresent invention). Generally speaking, the (polyimide) base polymerwill be at least 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90,95 or 100 weight percent of the overall adhesive composition of thepresent invention.

The base polymer of the present invention is a polyimide synthesized bya poly-condensation reaction, involving the reaction of one or morearomatic dianhydrides with at least two types of diamines—aromaticdiamine and aliphatic diamine. Substantially all of the diamine iseither aliphatic or aromatic, and the mole ratio of aliphatic diamine toaromatic diamine is A:B, where A is a range of from about 50, 55, 60, or65 to about 70, 75, 80, 85 or 90, and B is a range of from about 10, 15,20 or 25 to about 30, 35, 40, 45 or 50.

As used herein, an “aromatic diamine” is intended to mean a diaminehaving at least one aromatic ring, either alone (i.e., a substituted orunsubstituted, functionalized or unfunctionalized benzene orsimilar-type aromatic ring) or connected to another (aromatic oraliphatic) ring, and such an amine is to be deemed aromatic, regardlessof any non-aromatic moieties that might also be a component of thediamine. Hence, an aromatic diamine backbone chain segment is intendedto mean at least one aromatic moiety between two adjacent imidelinkages. As used herein, an “aliphatic diamine” is intended to mean anyorganic diamine that does not meet the definition of an aromaticdiamine.

Depending upon context, “diamine” as used herein is intended to mean:(i) the unreacted form (i.e., a diamine monomer); (ii) a partiallyreacted form (i.e., the portion or portions of an oligomer or otherpolyimide precursor derived from or otherwise attributable to diaminemonomer) or (iii) a fully reacted form (the portion or portions of thepolyimide derived from or otherwise attributable to diamine monomer).The diamine can be functionalized with one or more moieties, dependingupon the particular embodiment selected in the practice of the presentinvention.

Indeed, the term “diamine” is not intended to be limiting (orinterpreted literally) as to the number of amine moieties in the diaminecomponent. For example, (ii) and (iii) above include polymeric materialsthat may have two, one, or zero amine moieties. Alternatively, thediamine may be functionalized with additional amine moieties (inaddition to the amine moieties at the ends of the monomer that reactwith dianhydride to propagate a polymeric chain). Such additional aminemoieties could be used to crosslink the polymer or to provide otherfunctionality to the polymer.

Similarly, the dianhydride as used herein is intended to mean thecomponent that reacts with (is complimentary to) the diamine and incombination is capable of reacting to form an intermediate polyamic acid(which can then be cured into a polyimide). Depending upon context,“anhydride” as used herein can mean not only an anhydride moiety per se,but also a precursor to an anhydride moiety, such as: (i) a pair ofcarboxylic acid groups (which can be converted to anhydride by ade-watering or similar-type reaction); or (ii) an acid halide (e.g.,chloride) ester functionality (or any other functionality presentlyknown or developed in the future which is) capable of conversion toanhydride functionality.

Depending upon context, “dianhydride” can mean: (i) the unreacted form(i.e. a dianhydride monomer, whether the anhydride functionality is in atrue anhydride form or a precursor anhydride form, as discussed in theprior above paragraph); (ii) a partially reacted form (i.e., the portionor portions of an oligomer or other partially reacted or precursorpolyimide composition reacted from or otherwise attributable todianhydride monomer) or (iii) a fully reacted form (the portion orportions of the polyimide derived from or otherwise attributable todianhydride monomer).

The dianhydride can be functionalized with one or more moieties,depending upon the particular embodiment selected in the practice of thepresent invention. Indeed, the term “dianhydride” is not intended to belimiting (or interpreted literally) as to the number of anhydridemoieties in the dianhydride component. For example, (i), (ii) and (iii)(in the paragraph above) include organic substances that may have two,one, or zero anhydride moieties, depending upon whether the anhydride isin a precursor state or a reacted state. Alternatively, the dianhydridecomponent may be functionalized with additional anhydride type moieties(in addition to the anhydride moieties that react with diamine toprovide a polyimide). Such additional anhydride moieties could be usedto crosslink the polymer or to provide other functionality to thepolymer.

Ordinary skill and experimentation may be necessary in preparing thepolyimide compositions of the present invention, depending upon theparticular monomers selected and the particular polyimide manufacturingprocess selected in the practice of the present invention. In oneembodiment, the adhesive compositions of the present invention arepolymerized to a sufficient viscosity and cured to a sufficient degreeto provide the following properties:

-   -   A. z-axis dimensional stability (a z-axis coefficient of thermal        expansion factor) of less than 80, 85, 90, 95, 100, 105, 110,        115, 120, 125, 130, 135, 140, 145, or 150 ppm/° C. (ASTM Method        IPC-650 2.4.41),    -   B. a glass transition temperature from about 150, 160, 170, 180,        or 185 to about 190, 195, 197 or 200° C.; and    -   C. a modulus from 1, 5, 10, 25, 50, 75, or 100 to about 125,        150, 175 or 200, 250, 300, 400, 500, 600, 700, 800, 900, 1000,        1200 or more kpsi.

It would be impossible to discuss or describe all possible polyimidemanufacturing processes useful in the practice of the present invention.It should be appreciated that the monomer systems of the presentinvention are capable of providing the above-described advantageousproperties in a variety of manufacturing processes. The compositions ofthe present invention can be manufactured as described herein and can bereadily manufactured in any one of many (perhaps countless) ways ofthose of ordinarily skilled in the art, using any conventional ornon-conventional polyimide manufacturing technology.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one embodiment, the polyimide adhesives of the present invention areprepared by reacting a mixture of aliphatic and aromatic diamines with amixture of aromatic dianhydrides in an organic solvent, as follows.

I. Organic Solvents

Useful organic solvents for the synthesis of the low Tg polyimides ofthe present invention are preferably capable of dissolving the polyimideprecursor materials. Such a solvent should also have a relatively lowboiling points, such as below 225° C., so the polyimide can be dried atmoderate (i.e., more convenient and less costly) temperatures. A boilingpoint of less than 210, 205, 200, 195, 190, or 180° C. is preferred.

Solvents of the present invention may be used alone or in combinationwith other solvents (i.e., cosolvents). Useful organic solvents include:N-methylpyrrolidone (NMP), dimethylacetamide (DMAc),N,N′-dimethyl-formamide (DMF), dimethyl sulfoxide (DMSO), tetramethylurea (TMU), diethyleneglycol diethyl ether, 1,2-dimethoxyethane(monoglyme), diethylene glycol dimethyl ether (diglyme),1,2-bis-(2-methoxyethoxy) ethane (triglyme), bis [2-(2-methoxyethoxy)ethyl)] ether (tetraglyme), gamma-butyrolactone, andbis-(2-methoxyethyl) ether, tetrahydrofuran. In one embodiment,preferred solvents include N-methylpyrrolidone (NMP) anddimethylacetamide (DMAc).

Co-solvents can generally be used at about 5 to 50 weight percent of thetotal solvent, and useful such co-solvents include xylene, toluene,benzene, “Cellosolve” (glycol ethyl ether), and “Cellosolve acetate”(hydroxyethyl acetate glycol monoacetate).

II. Aliphatic Diamines

In one embodiment, useful aliphatic diamines have the followingstructural formula: H₂N—R—NH₂, where R is an aliphatic moiety, such as asubstituted or unsubstituted hydrocarbon in a range from 4, 5, 6, 7 or 8carbons to about 9, 10, 11, 12, 13, 14, 15, or 16 carbon atoms, and inone embodiment the aliphatic moiety is a C₆ to C₈ aliphatic.

In one embodiment, R is a C₆ straight chain hydrocarbon, known ashexamethylene diamine (HMD or 1,6-hexanediamine). In other embodiments,the aliphatic diamine is an alpha, omega-diamine; such diamines can bemore reactive than alpha, beta-aliphatic diamines.

In one embodiment, to achieve low temperature bonding (“low temperaturebonding” is intended to mean bonding in a range of from about 180, 185,or 190° C. to about 195, 200, 205, 210, 215, 220, 225, 230, 235, 240,245 and 250° C.), the mole % of aliphatic diamine (based upon totaldiamine) is in a range from about 50, 55, 60, 65, or 70 to about 75, 80,85 or 90 mole %. In this embodiment, if less than 50 mole % of thediamine component is aliphatic diamine, the resulting polyimide cansometimes have an unduly high glass transition temperature (“T_(g)”)which can be detrimental to low temperature bonding. In one embodiment,if more than 90 mole % of the diamine component is an aliphatic diamine,then any resulting polyimide film can become too brittle for someflexible material applications.

In one embodiment, as the aliphatic diamine to aromatic diamine ratioincreases, the glass transition temperature (Tg) of the polyimide, andlamination temperature will generally tend to decrease. In oneembodiment, for bonding to metal to properly occur, the laminationtemperature is typically about 25° C. higher than the glass transitiontemperature of the polyimide adhesive. For example, if the glasstransition temperature of the polyimide is in the range of about 150° C.to 200° C., then the optimal bonding temperature will be in the range ofabout 180° C. to 250° C.

In one embodiment, the aliphatic diamine is 75±10, 8, 6, 4, 2 or 1 mole% hexamethylene diamine (HMD) and the aromatic diamine is 25±10, 8, 6,4, 2 or 1 mole %, 1,3-bis-(4-aminophenoxy) benzene (APB-134, RODA).Here, the glass transition temperature of the polyimide adhesive is in arange of about 175±10° C. At a lamination temperature (bondingtemperature) of about 200±10, 8, 6, 4, 2 or 1° C., the polyimideadhesive can be a viable substitute for an acrylic or epoxy coverlaycomposition, compositions commonly used as conformal coatings andencapsulates in electronics applications.

Depending upon the particular embodiment of the present invention, othersuitable aliphatic diamines include, 1,4-tetramethylenediamine,1,5-pentamethylenediamine (PMD), 1,7-heptamethylene diamine,1,8-octamethylenediamine, 1,9-nonamethylenediamine,1,10-decamethylenediamine (DMD), 1,11-undecamethylenediamine,1,12-dodecamethylenediamine (DDD), 1,16-hexadecamethylenediamine. Thepreferred aliphatic diamine is hexamethylene diamine (HMD).

III. Aromatic Diamines

In one embodiment, from about 5, 10, 15, 20, or 25 mole % to about 30,35, 40, 45, and above, but less than 50 mole % of the diamine componentof the polyimide adhesives of the present invention are aromaticdiamines. Other suitable aromatic diamines include, m-phenylenediamine,p-phenylenediamine, 2,5-dimethyl-1,4-diaminobenzene,trifluoromethyl-2,4-diaminobenzene, trifluoromethyl-3,5-diaminobenzene,2,5-dimethyl-1,4-phenylenediamine (DPX), 2,2-bis-(4-aminophenyl)propane, 4,4′-diaminobiphenyl, 4,4′-diaminobenzophenone,4,4′-diaminodiphenylmethane, 4,4′-diaminodiphenyl sulfide,4,4′-diaminodiphenyl sulfone, 3,3′-diaminodiphenyl sulfone,bis-(4-(4-aminophenoxy)phenyl sulfone (BAPS),4,4′-bis-(aminophenoxy)biphenyl (BAPB), 4,4′-diaminodiphenyl ether,3,4′-diaminodiphenyl ether, 4,4′-diaminobenzophenone,4,4′-isopropylidenedianiline, 2,2′-bis-(3-aminophenyl)propane,N,N-bis-(4-aminophenyl)-n-butylamine, N,N-bis-(4-aminophenyl)methylamine, 1,5-diaminonaphthalene, 3,3′-dimethyl-4,4′-diaminobiphenyl,m-amino benzoyl-p-amino anilide, 4-aminophenyl-3-aminobenzoate,N,N-bis-(4-aminophenyl) aniline, 2,4-diaminotoluene, 2,5-diaminotoluene,2,6-diaminotoluene, 2,4-diamine-5-chlorotoluene,2,4-diamine-6-chlorotoluene, 2,4-bis-(beta-amino-t-butyl) toluene,bis-(p-beta-amino-t-butyl phenyl) ether,p-bis-2-(2-methyl-4-aminopentyl) benzene, m-xylylene diamine, andp-xylylene diamine.

Other useful aromatic diamines include, 1,2-bis-(4-aminophenoxy)benzene,1,3-bis-(4-aminophenoxy) benzene, 1,2-bis-(3-aminophenoxy)benzene,1,3-bis-(3-aminophenoxy) benzene, 1-(4-aminophenoxy)-3-(3-aminophenoxy)benzene, 1,4-bis-(4-aminophenoxy) benzene, 1,4-bis-(3-aminophenoxy)benzene, 1-(4-aminophenoxy)-4-(3-aminophenoxy) benzene,2,2-bis-(4-[4-aminophenoxy]phenyl) propane (BAPP),2,2′-bis-(4-aminophenyl)-hexafluoro propane (6F diamine),2,2′-bis-(4-phenoxy aniline) isopropylidene,2,4,6-trimethyl-1,3-diaminobenzene, 4,4′-diamino-2,2′-trifluoromethyldiphenyloxide, 3,3′-diamino-5,5′-trifluoromethyl diphenyloxide,4,4′-trifluoromethyl-2,2′-diaminobiphenyl,2,4,6-trimethyl-1,3-diaminobenzene,4,4′-oxy-bis-[2-trifluoromethyl)benzene amine] (1,2,4-OBABTF),4,4′-oxy-bis-[3-trifluoromethyl)benzene amine],4,4′-thio-bis-[(2-trifluoromethyl)benzene-amine],4,4′-thiobis[(3-trifluoromethyl)benzene amine],4,4′-sulfoxyl-bis-[(2-trifluoromethyl)benzene amine,4,4′-sulfoxy]-bis-[(3-trifluoromethyl)benzene amine], and4,4′-keto-bis-[(2-trifluoromethyl)benzene amine].

In one embodiment, the preferred aromatic diamines are the isomers ofbis-aminophenoxybenzenes (APB), aminophenoxyphenylpropane (BAPP),dimethylphenylenediamine (DPX), bisaniline P, and combinations thereof.In certain embodiments, the use of these particular diamines can lowerthe lamination temperature of the adhesive, and will increase the peelstrength of the adhesive to other materials, especially metals.

IV. Aromatic Dianhydrides

In this embodiment, any aromatic dianhydride or combination of aromaticdianhydrides, can be used as the dianhydride monomer in forming thepolyimide. These dianhydrides may be used alone or in combination withone another. The dianhydrides can be used in their tetra-acid form (oras mono, di, tri, or tetra esters of the tetra acid), or as theirdiester acid halides (chlorides). However in some embodiments, thedianhydride form can be preferred, because it is generally more reactivethan the acid or the ester.

Examples of suitable aromatic dianhydrides include, 1,2,5,6-naphthalenetetracarboxylic dianhydride, 1,4,5,8-naphthalene tetracarboxylicdianhydride, 2,3,6,7-naphthalene tetracarboxylic dianhydride,2-(3′,4′-dicarboxyphenyl) 5,6-dicarboxybenzimidazole dianhydride,2-(3′,4′-dicarboxyphenyl) 5,6-dicarboxybenzoxazole dianhydride,2-(3′,4′-dicarboxyphenyl) 5,6-dicarboxybenzothiazole dianhydride,2,2′,3,3′-benzophenone tetracarboxylic dianhydride,2,3,3′,4′-benzophenone tetracarboxylic dianhydride,3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA),2,2′,3,3′-biphenyl tetracarboxylic dianhydride, 2,3,3′,4′-biphenyltetracarboxylic dianhydride, 3,3′,4,4′-biphenyl tetracarboxylicdianhydride (BPDA),bicyclo-[2,2,2]-octen-(7)-2,3,5,6-tetracarboxylic-2,3,5,6-dianhydride,4,4′-thio-diphthalic anhydride, bis (3,4-dicarboxyphenyl) sulfonedianhydride, bis (3,4-dicarboxyphenyl) sulfoxide dianhydride (DSDA), bis(3,4-dicarboxyphenyl oxadiazole-1,3,4) p-phenylene dianhydride, bis(3,4-dicarboxyphenyl) 2,5-oxadiazole 1,3,4-dianhydride, bis2,5-(3′,4′-dicarboxydiphenylether) 1,3,4-oxadiazole dianhydride,4,4′-oxydiphthalic anhydride (ODPA), bis (3,4-dicarboxyphenyl) thioether dianhydride, bisphenol A dianhydride (BPADA), bisphenol Sdianhydride, 2,2-bis-(3,4-dicarboxyphenyl)1,1,1,3,3,3,-hexafluoropropane dianhydride (6FDA),5,5-[2,2,2]-trifluoro-1-(trifluoromethyl)ethylidene,bis-1,3-isobenzofurandione, 1,4-bis(4,4′-oxyphthalic anhydride) benzene,bis (3,4-dicarboxyphenyl) methane dianhydride, cyclopentadienyltetracarboxylic acid dianhydride, cyclopentane tetracarboxylicdianhydride, ethylene tetracarboxylic acid dianhydride, perylene3,4,9,10-tetracarboxylic dianhydride, pyromellitic dianhydride (PMDA),tetrahydrofuran tetracarboxylic dianhydride, 1,3-bis-(4,4′-oxydiphthalicanhydride) benzene, 2,2-bis(3,4-dicarboxyphenyl) propane dianhydride,2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride,2,7-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride,2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic dianhydride,phenanthrene-1,8,9,10-tetracarboxylic dianhydride,pyrazine-2,3,5,6-tetracarboxylic dianhydride,benzene-1,2,3,4-tetracarboxylic dianhydride; andthiophene-2,3,4,5-tetracarboxylic dianhydride.

In this embodiment, the preferred dianhydrides are BTDA and BPDA as theyare readily available and have been found to provide excellentproperties.

V. Preparation of the Polyimide Adhesives

A polyimide film according to the present invention can be produced bycombining the diamine and dianhydride (monomer or other polyimideprecursor form) together with a solvent to form a polyamic acid (alsocalled a polyamide acid) solution. The dianhydride and diamine can becombined in a molar ratio of about 0.90 to 1.10. Molecular weight of thepolyamic acid formed therefrom can be adjusted by adjusting the molarratio of the dianhydride and diamine.

In one embodiment, a polyamic acid casting solution is derived from thepolyamic acid solution. The polyamic acid casting solution preferablycomprises the polyamic acid solution combined with conversion chemicalslike: (i.) one or more dehydrating agents, such as, aliphatic acidanhydrides (acetic anhydride, etc.) and/or aromatic acid anhydrides; and(ii.) one or more catalysts, such as, aliphatic tertiary amines(triethyl amine, etc.), aromatic tertiary amines (dimethyl aniline,etc.) and heterocyclic tertiary amines (pyridine, picoline,isoquinoline, etc.). The anhydride dehydrating material it is often usedin molar excess compared to the amount of amide acid groups in thepolyamic acid. The amount of acetic anhydride used is typically about2.0-3.0 moles per equivalent of polyamic acid. Generally, a comparableamount of tertiary amine catalyst is used.

In one embodiment, the polyamic acid solution, and/or the polyamic acidcasting solution, is dissolved in an organic solvent at a concentrationfrom about 5.0 or 10% to about 15, 20, 25, 30, 35 and 40% by weight.

The polyamic acid (and casting solution) can further comprise any one ofa number of additives, such as processing aids (e.g., oligomers),antioxidants, light stabilizers, flame retardant additives, anti-staticagents, heat stabilizers, ultraviolet absorbing agents, inorganicfillers or various reinforcing agents. These inorganic fillers includethermally conductive fillers, like metal oxides, and electricallyconductive fillers like metals and electrically conductive polymers.Common inorganic fillers are alumina, silica, silicon carbide, diamond,clay, boron nitride, aluminum nitride, titanium dioxide, dicalciumphosphate, and fumed metal oxides. Common organic fillers includepolyaniline, polythiophene, polypyrrole, polyphenylenevinylene,polydialkylfluorenes, carbon black, and graphite.

The solvated mixture (the polyamic acid casting solution) can then becast or applied onto a support, such as an endless belt or rotatingdrum, to give a film. Next, the solvent containing-film can be convertedinto a self-supporting film by baking at an appropriate temperature(thermal curing) together with conversion chemical reactants (chemicalcuring). The film can then be separated from the support, oriented suchas by tentering, with continued thermal and chemical curing to provide apolyimide film.

Useful methods for producing polyimide film in accordance with thepresent invention can be found in U.S. Pat. No. 5,166,308 and U.S. Pat.No. 5,298,331 are incorporate by reference into this specification forall teachings therein. Numerous variations are also possible, such as,

(a) A method wherein the diamine components and dianhydride componentsare preliminarily mixed together and then the mixture is added inportions to a solvent while stirring.

(b) A method wherein a solvent is added to a stirring mixture of diamineand dianhydride components. (contrary to (a) above)

(c) A method wherein diamines are exclusively dissolved in a solvent andthen dianhydrides are added thereto at such a ratio as allowing tocontrol the reaction rate.

(d) A method wherein the dianhydride components are exclusivelydissolved in a solvent and then amine components are added thereto atsuch a ratio to allow control of the reaction rate.

(e) A method wherein the diamine components and the dianhydridecomponents are separately dissolved in solvents and then these solutionsare mixed in a reactor.

(f) A method wherein the polyamic acid with excessive amine componentand another polyamic acid with excessive dianhydride component arepreliminarily formed and then reacted with each other in a reactor,particularly in such a way as to create a non-random or block copolymer.

(g) A method wherein a specific portion of the amine components and thedianhydride components are first reacted and then the residual diaminecomponents are reacted, or vice versa.

(h) A method wherein the conversion chemicals are mixed with thepolyamic acid to form a polyamic acid casting solution and then cast toform a gel film.

(i.) A method wherein the components are added in part or in whole inany order to either part or whole of the solvent, also where part or allof any component can be added as a solution in part or all of thesolvent.

(j) A method of first reacting one of the dianhydride components withone of the diamine components giving a first polyamic acid. Thenreacting the other dianhydride component with the other amine componentto give a second polyamic acid. Then combining the amic acids in any oneof a number of ways prior to film formation.

The thickness of the polyimide film may be adjusted depending on theintended purpose of the film or final application specifications. It isgenerally preferred that the thickness of the film ranges from 2, 3, 5,7, 8, 10, 12, 15, 20, or 25 microns to about 25, 30, 35, 40, 45, 50, 60,80, 100, 125, 150, 175, 200, 300, 400 or 500 microns. Preferably, thethickness is from about 8 to about 125 microns, more preferably from 12to 25 microns.

Polyimide films according to the present invention can be used as a basefilm adhesive for a laminate for incorporation into a flexible printedcircuit board (“FPC”). In one embodiment, a flexible printed circuitboard (“FPC”) can be produced as follows:

-   -   1. laminating a copper or other conductive foil (or conductive        layer) to the adhesive polyimide;    -   2. forming a circuit pattern (broadly speaking: application of a        resist, photo-patterning and development of the resist, copper        etching and removal of the resist).

In one embodiment, the films of the present invention are used as acoverlay film. Coverlay films are laminated to etched circuitry traces(metal traces) of a flexible printed circuit board. The adhesivepolyimide encapsulates the copper circuitry, protecting it from theenvironment and providing electrical and thermal insulation. Theflexible printed circuit board, covered with the films of the presentinvention, may be single sided, double sided, or be incorporated into astack of several individual flexible printed circuits to form what iscommonly referred to as a multilayer board. Any of these types ofcircuits may be used in a solely flexible printed circuit or may becombined with rigid circuitry applications to form a rigid/flex orflex/rigid printed wiring board.

The polyimide films of the present invention may have other polyimidesbonded to them. Examples of such polyimides are higher T_(g) polyimidesused as a base layer in a two or three-layer, co-extruded product. Insuch a co-extruded product, the polyimide adhesives of the presentinvention are simultaneously solution cast with high-modulus polyimides.At the time of casting, the polyimides are in the form of a polyamicacid solution. The cast solutions form an uncured polyamic acid filmthat is later cured to a polyimide. In one embodiment, the polyimideadhesives are cast with a single layer of higher T_(g) polyimide.

In another embodiment, the polyimide adhesives of the present inventionare cast on two sides of a higher T_(g) polyimide. In yet anotherembodiment, the polyimides of the present invention are cast alone toform a sheet adhesive material useful as a center layer in a metallaminate structure, or as a coverlay material for a printed circuitboard.

The adhesion strength of the above-described laminates can be improvedby employing various techniques for elevating adhesion strength. Forexample, prior to the step of applying the adhesives of the presentinvention onto a metal foil, or the exposed circuitry in a coverlayapplication, the polyimide can be subjected to a pre-treatment step.These pre-treatment steps include, heat treatment, corona treatment,plasma treatment under atmospheric pressure, plasma treatment underreduced pressure, treatment with coupling agents like silanes andtitanates, sandblasting, alkali-treatment, acid-treatments, and coatingpolyamic acids. To improve the adhesion strength, it is generally alsopossible to add various metal compounds as disclosed in U.S. Pat. Nos.4,742,099; 5,227,244; 5,218,034; and 5,543,222 incorporated herein byreference.

In addition, to improve adhesion between the adhesives of the presentinvention and metal foil, or exposed circuitry in a coverlayapplication, the metal surface may be treated with various organic andinorganic treatments. These treatments include using silanes,imidazoles, triazoles, oxide and reduced oxide treatments, tin oxidetreatment, and surface cleaning/roughening (called micro-etching) viaacid or alkaline reagents.

The polyimide adhesives of the present invention can also be applied tofully cured polyimide base films or can be applied to one of theintermediate manufacturing stages of polyimide film such as to “gel”film or to “green” film.

The term “gel” film refers to a polyamic acid sheet, which is laden withvolatiles, primarily solvent, to such an extent that the polyamic acidis in a gel-swollen, or rubbery condition. The volatile content isusually in the range of 70 to 90% by weight and the polymer contentusually in the range of 10 to 30% by weight of the gel film. The finalfilm becomes “self-supporting” in the gel film stage. It can be strippedfrom the support on which it was cast and heated to a final curingtemperature. The gel film generally has an amic acid to imide ratiobetween 10:90 and 50:50, most often 30:70.

The gel film structure can be prepared by the method described in U.S.Pat. No. 3,410,826. This prior art discloses mixing a chemicalconverting agent and a catalyst such as a lower fatty acid anhydride anda tertiary amine, into the polyamic-acid solution at a low temperature.This is followed by casting the polyamic-acid solution in film-form,onto a casting drum. The film is mildly heated after casting, at forexample 100° C., to activate the conversion agent and catalyst in orderto transform the cast film to a polyamic acid/polyimide gel film.

Another type of polyimide base film, is a “green film” which ispartially polyamic acid and partially polyimide. Green film containsgenerally about 50 to 75% by weight polymer and 25 to 50% by weightsolvent. It is sufficiently strong to be self-supporting. Green film canbe prepared by casting the polyamic acid solution into film form onto asuitable support such as a casting drum or belt and removing the solventby mild heating at up to 150° C. A low proportion of amic acid units inthe polymer, e.g., up to 25%, may be converted to imide units.

Application of the adhesives of the present invention can beaccomplished in any number of ways. Such methods include using a slotdie, dip coating, or kiss-roll coating a film followed by metering withdoctor knife, doctor rolls, squeeze rolls, or an air knife. The coatingmay also be applied by brushing or spraying. By using such techniques,it is possible to prepare both one and two-sided coated laminates. Inpreparation of the two-side coated structures, one can apply thecoatings to the two sides of a polyimide either simultaneously orconsecutively before going to the curing and drying stage of thepolyimide.

In a further embodiment, the polyamic acid adhesive may be coated on afully cured polyimide base film or directly on a metal substrate andsubsequently imidized by heat treatment. The polyimide base film may beprepared by either a chemical or thermal conversion process and may besurface treated, e.g. by chemical etching, corona treatment, laseretching etc., to improve adhesion.

A single polyimide metal-clad of the present invention comprises aflexible polyimide layer which adheres to a metal foil such as copper,aluminum, nickel, steel or an alloy containing one or more of thesemetals.

The polyimide layer adheres firmly to the metal and has a peel strengthof 2 pounds per linear inch and higher. The metal may be adhered to oneor both sides of the polyimide layer.

The polyimide adhesive films of the present invention will bond tocopper at from about 2 pounds per linear inch to about 15 pounds perlinear inch. The bonding temperature is typically between 180° C. and250° C. In one embodiment, a polyimide adhesive of the present inventionbonded to copper with a bonding strength of about 8 pounds per linearinch, a bonding temperature of 200° C., and a glass transitiontemperature of about 165 to 185° C.

As used herein, the term “conductive layers” and “conductive foils” aremeant to be metal layers or metal foils. Conductive foils are typicallymetal foils. Metal foils do not have to be used as elements in pureform; they may also be used as metal foil alloys, such as copper alloyscontaining nickel, chromium, iron, and other metals. The conductivelayers may also be alloys of metals and are typically applied to thepolyimides of the present invention via a sputtering step followed by anelectroplating step. In these types of processes, a metal seed coatlayer is first sputtered onto the polyimide adhesive. Finally, a thickercoating of metal is applied to the seed coat via electro-plating orelectro-deposition. Such sputtered metal layers may also be hot pressedabove the glass transition temperature of the polymer for enhanced peelstrength.

Conductive foils are also useful. Particularly suitable metallicsubstrates are foils of rolled, annealed copper or rolled, annealedcopper alloy. In many cases, it has proved to be of advantage topre-treating the metallic substrate before coating. This pretreatmentmay include, but is not limited to, electro-deposition orimmersion-deposition on the metal of a thin layer of copper, zinc,chrome, tin, nickel, cobalt, other metals, and alloys of these metals.The pretreatment may consist of a chemical treatment or a mechanicalroughening treatment. It has been found that this pretreatment enablesthe adhesion of the polyimide layer and, hence, the peel strength to befurther increased. Apart from roughening the surface, the chemicalpretreatment may also lead to the formation of metal oxide groups,enabling the adhesion of the metal to the polyimide layer to be furtherincreased. This pretreatment may be applied to both sides of the metal,enabling enhanced adhesion to substrates on both sides.

A polyimide multi-clad of the present invention comprising a double sidecopper clad can be prepared by laminating copper foil to both sides ofan adhesive coated dielectric polyimide film. The construction can alsobe made by laminating adhesive coated copper foil to both sides of adielectric polyimide film or to an adhesive coated dielectric polyimidefilm.

The polyimides of the present invention provide many attributes notcommon to materials that bond in the temperature range of from about180° C. to 250° C. Firstly, the polyimide adhesives of the presentinvention provide superior thermal resistance and z-axis dimensionalstability. In thin film or laminate construction, dimensional stabilityis measured in ppm/° C. and is typically referred to with respect to thex-y plane. However, in modern circuitry applications z-directionaldimensional stability (through the film thickness), especially in filmswhere the dielectric is less than 25 microns, is increasingly important.The films of the present invention are superior in z-axis dimensionalstability (z-axis coefficient of thermal expansion) by providing aexpansion factor of less than 120 ppm/° C., typically 90 ppm/° C.,whereas most acrylics are about 300 to 400 ppm/° C. The coefficient ofthermal expansion is measured by ASTM Method IPC-650 2.4.41 and isincluded herein by reference.

In addition to providing superior z-axis dimensional stability(coefficient of thermal expansion) the polyimide adhesive films of thepresent invention have a low loss-tangent value. Loss-tangent istypically measured at 10 GHz and is used to measure a dielectricmaterial's degradation of a nearby digital signal that is passingthrough a metal circuit trace. Different loss-tangent values exist fordifferent dielectric materials. The lower the loss-tangent value for agiven dielectric material, the more superior a material is for digitalcircuitry applications. The polyimide adhesives of the present inventionexhibit superior, low loss-tangent values. In one embodiment, theloss-tangent value for the polyimide adhesive was less than 0.010, about0.004, at 10 GHz. The polyimides of present invention may also be usedin applications ranging from 1 to 100 GHz, with 1 to 20 GHz being mostcommon. For acrylic and epoxy dielectric materials, poor loss-tangentsvalues of about 0.025 at 10 GHz are typically observed.

In another embodiment, the polyimides of the present invention are usedas a material used to construct a planar transformer component. Theseplanar transformer components are commonly used in power supply devices.In yet another embodiment, the polyimide adhesives of the presentinvention may be used with thick metal foils (like Inconel) to formflexible heaters. These heaters are typically used in automotive andaerospace applications.

The polyimide films of the present invention exhibit superiorattenuation when compared to the acrylics and epoxies of the prior art.The polyimide adhesives of the present invention exhibit an attenuationvalue, measured in decibels per inch, of about 0.3 at 10 GHz using a50-ohm micro strip. Acrylic materials under the same test exhibit anattenuation value of about 0.9. As such, the polyimide adhesives of thepresent invention are superior over acrylics and epoxies due to theirsuperior z-axis dimensional stability, low loss-tangent, and lowattenuation.

In one embodiment of the present invention, the polyimide adhesives areused in combination with higher Tg polyimides to form polyimidemetal-clad laminates. As used herein, higher Tg polyimides arepolyimides that are widely considered as thermosetting polyimides,commonly referred to as “thermoset” polyimides. These polyimides arederived from dianhydrides such as PMDA, BPDA, BTDA and the like, anddiamines such as p-phenylene diamine, m-phenylene diamine,3,4′-oxydianiline, 4,4′-oxydianiline, and biphenyldiamine. The higherT_(g) polyimides films mentioned above are preferably about 0.3 to 5.0mils in thickness. These higher Tg polyimides can be obtained frompolyamic acid precursors derived from the reaction of suitable diamineswith suitable dianhydrides in the manner described in, for example, U.S.Pat. Nos. 3,179,630, 3,179,633, 3,179,634, 5,166,308, and 5,196,500incorporated herein by reference.

In one embodiment, the adhesive polyimides of the present invention andthe higher Tg polyimides mentioned above, are cast in their polyamicacids precursor forms using a multi-port die to form either two layerand three layer polyimides. These multi-layer polyimides are then bondedto metal using the polyimide adhesive as the bonding medium to themetal. Thus, the polyimide film metal-clad laminates formed comprise atleast one layer of a polyimide base film (the high modulus layer) and atleast one layer of polyimide adhesive film (the films herein of thepresent invention).

Bonding of the multi-layer polyimide to the metal usually takes place ina double belt press in roll to roll processing, or in an autoclave insheet to sheet processing. Alternatively, the polyimide adhesives of thepresent invention may be directly bonded to one or both sides of a metalsubstrate to form a polyimide metal-clad laminate. Yet, another methodof forming a polyimide film metal-clad laminate is to coat an alreadyformed higher T_(g) and/or high modulus polyimide with the polyimideadhesives of the present invention and then bonded the multi-layerpolyimide to a metal. Finally, yet another way to form the polyimidemetal-clad laminate is to sputter a metal seed coat and thenelectro-plate a thicker metal layer onto an existing polyimide adhesivelayer, or polyimide multi-layer.

Thus, a polyimide multi-clad of the present invention comprises at leastone layer metal and one layer of the polyimide adhesives of the presentinvention. In some cases, a higher Tg and/or high modulus polyimide isalso incorporated and in other cases two layers of metal, and two layersof polyimide adhesive may be employed.

The polyimide films of the present invention can be used as an adhesivefilm, optionally with a higher T_(g) polyimide film. Higher T_(g)polyimide films for purposes of this invention are meant to be polyimidefilms that either have no measurable glass transition temperature, orhave a glass transition temperature greater than 250° C. Thesepolyimides are used for insulating electronic parts, electronic circuitboards, and electronic equipment.

The films of the present invention, when used with higher T_(g)polyimide films, are particularly useful for die pad bonding of flexibleprint connection boards or semiconductor devices or packaging materialsfor CSP (chip scale package), chip on flex (COF), COL (chip on lead),LOC (lead on chip), multi-chip module (“MCM”), ball grid array (“BGA” ormicro-ball grid array), and/or tape automated bonding (“TAB”).

In one embodiment, the present invention includes a method for bondingan integrated circuit chip to a lead frame. The method includes firstpreparing a solution in an organic solvent of a polyamic acid comprisingthe reaction product of components comprising an aromatic dianhydrideand a diamine. The diamine consists essentially of a mixture of about 50to about 90 mole % of an aliphatic diamine and about 10 to about 50 mole% of an aromatic diamine. Next, the polyamic acid is applied to eitherthe integrated circuit chip or the lead frame. Under heat, the organicsolvent is removed and the polyamic acid is converted via imidization toa polyimide. Then, under pressure and heat, the integrated circuit chipand lead frame are bonded together.

In another embodiment, the polyimide adhesive films of the presentinvention are used for wafer level integrated circuit packaging, where acomposite is made comprising a substrate according to the presentinvention interposed between a conductive layer (typically a metal)having a thickness of less than 100 microns, and a wafer comprising aplurality of integrated circuit dies. Here, the adhesives of the presentinvention are used as the adhesive layer that bonds the conductive layerto high modulus, polyimide substrates. In one (wafer level integratedcircuit packaging) embodiment, the conductive passageway is connected tothe dies by a conductive passageway, such as a wire bond, a conductivemetal, a solder bump or the like.

The advantageous properties of this invention can be observed byreference to the following examples that illustrate, but do not limit,the invention. All parts and percentages are by weight unless other wiseindicated.

EXAMPLES

Polyamic acid solutions were prepared reacting the appropriate molarequivalents of the monomers in dimethylacetamide (DMAc) solvent.Typically, the diamine dissolved in DMAc was stirred under nitrogen, andthe dianhydride was added as a solid over a period of several minutes.Stirring was continued to obtain maximum viscosity of the polyamic acid.The viscosity was adjusted by controlling the amount of dianhydride inthe polyamic acid composition.

The polyamic acids were coated on a fully cured corona treated polyimidebase film derived from pyromellitic dianhydride and 4,4′-sdiaminodiphenyl ether. The polyamic acids were converted to polyimideeither by a thermal conversion process or by a chemical conversionprocess using acetic anhydride as a dehydrating agent and beta-picolineas a catalyst.

The polyamic acids were coated on the base polyimide film using acoating bar to a thickness of 0.5 mils and the solvent removed byheating, although coating thicknesses of from about 5, 7.5, 10, 12.5,15, 17.5, 20, 22.5 or 25 to about 30, 35, 40, 45, 50, 60, 70, 80 or moremicrons can be appropriate, depending upon the particular embodiment.The coated polyimide films were placed on a pin frame and cured.

The coated polyimide films were subsequently laminated to roll-annealedcopper at temperatures of 180° C. to 250° C. to form a polyimide-metalclad.

Roll clad laminates could also be made by continuous lamination of theadhesive coated dielectric film to copper foil using a high temperaturedouble belt press or a high temperature nip roll laminator. Peelstrength results, of the polyimide copper-clad laminates, weredetermined by ASTM method IPC-TM-650, Method No. 2.4.9D.

Examples 1-9

1,3-bis-(4-aminophenoxy)benzene (APB-134) and 1,6-hexanediamine (HMD)were dissolved in dry dimethylacetamide (DMAc) solvent using a 1-literbeaker. The beaker was placed in a dry box. The mixture was stirred welland the temperature raised to 50° C.

A mixture of biphenyltetracarboxylic dianhydride (BPDA) and benzophenonetetracarboxylic dianhydride (BTDA) was prepared as the dianhydridemixture. Ninety-five percent by weight of the dianhydride mixture wasadded slowly to the diamine, over a period of 10 minutes.

The exothermic reaction was allowed to rise to 80° C. to ensure completereaction of the diamines and dianhydrides to form a polyamic acidsolution. The viscosity of the polyamic acid was adjusted, by adding aportion of the remaining dianhydride, to a desirable value anywhere from50 poise to 1000 poise. The polyamic acid solution was stirred for anadditional 1 hour at 35° C.

A small portion of the polyamic acid solution was cast on a glass plate.The casting was dried on a hot plate at 80° C. for 30 minutes. A two mil(two thousands of an inch) thick film was produced.

The film was peeled from the glass plate and placed on a steel pinframe. Then the film was dried (and partially cured) in a hightemperature oven. The starting temperature was 80° C., and temperaturewas increased to 250° C. at a rate of 5° C./min. The film was removedfrom the oven and cured for an additional 5.0 minutes in an oven set at330° C.

A copper laminate was made by placing the cured film against the treatedside of a 1 oz RA copper foil. Bonding took place at 180° C. to 250° C.,at 350-psi pressure, using vacuum lamination process. Thepolyimide-metal laminate was tested for adhesion strength using anInstron tester per ASTM Method IPC-TM-650, Method No. 2.4.9.D. Theseresults are giving in the Tables 1 and 2 below. The peel numbers weregenerally high at greater than 200° C. lamination, in the range of about2 pounds per linear inch (pli), commonly 8 pounds per linear inch.

The film was also used as coverlay and bond-ply. Coverlay compositionsare used to protect the delicate circuit traces (fragile metal circuitpatterns) that would otherwise be exposed on the surface of the flexiblecircuit or that would otherwise be susceptible to damage. Coverlaycompositions are placed over the circuit traces as a sheet, and thenvacuum-pressed and/or roll-pressed, so that the coverlay is bondeddirectly to the circuit traces. Good encapsulation of the copperconductor lines (circuit traces) occurred at 200° C. and 350 psipressure. A bond-ply is a layer of material used to bond a second layerof polyimide to either a copper layer or a rigid board assembly.Typically, a bond-ply material (like the adhesive polyimide of thepresent invention) is used on either side of another polyimide to encasethat polyimide within the adhesive structure of the outer two layers. Assuch, a second polyimide is encased within two polyimide adhesive layerswhere the inner layer is used as a highly dimensionally stable material,a thermal conductive polyimide, a high modulus polyimide, or simply alower cost polyimide. The coverlay laminates and bond-ply constructionsformed herein are suitable for manufacture of a high count, multi-layer,or rigid/flex printed wiring board assemblies. TABLE 1 LAMINATIONLAMINATION LAMINATION LAMINATION ADHESIVE TEMP 180 C. TEMP 190 C. TEMP200 C. TEMP 250 C. EX. COMPOSITION PEEL PEEL PEEL PEEL NO. (MOLE %)STRENGTH STRENGTH STRENGTH STRENGTH CURING Tg EX-1 BPDA/BTDA/RODA/HMD NOADHESION 2.7 PLI   6 PLI 8.5 PLI 70 C.-150 1 HR 181.8 90%/10%/35%/150-250 C. 65% 1 HR. 320 C. 5 MIN. EX-2 BPDA/BTDA/RODA/HMD 1.2 PLI   6PLI 7.4 PLI   9 PLI SAME 178.2 90%/10%/30%/ 70% EX-3 BPDA/BTDA/RODA/HMD1.5 PLI 4.5 PLI 6.8 PLI 6.5 PLI 70-150 C. 1 HR 164.7 90%/10%/20%/150-250 C. 1 HR 80% 300 C. 5 MIN. EX-4 BPDA/BTDA/RODA/HMD 1.8 PLI 6.5PLI   8 PLI   8 PLI 70-150-1 HR 175.2 90%/10%/25%/ 150-250 C. 1 HR 75%320 C. 5 MIN. EX-5 BPDA/BTDA/RODA/HMD   2 PLI   4 PLI   8 PLI   8 PLISAME 169.6 70%/30%/25%/ 75% EX-6 BPDA/BTDA/RODA/HMD 1.8 PLI 1.8 PLI 4.5PLI 6.8 PLI SAME 169.7 50%/50%/25%/ 75% EX-7 BPDA/BTDA/RODA/HMD NOADHESION 1.5 PLI 5.7 PLI 6.5 PLI SAME 168.1 30%/70%/25%/ 75% EX-8BPDA/BTDA/RODA/HMD NO ADHESION 1.5 PLI   4 PLI 6.3 PLI SAME 166.610%/90%/25%/ 75% EX-9 BPDA/BTDA/RODA/HMD NO ADHESION 3.5 PLI 4.5 PLI 5.6PLI SAME 160.2 0%/100%/25%/ 75%

Comparative Examples 1-7

The comparative examples below were preparing in accordance with theexamples. The same components, processing conditions, and procedureswere followed. However, the molar ratio of the components used wasaltered. TABLE 2 LAMINATION LAMINATION LAMINATION LAMINATION CompADHESIVE TEMP 180 C. TEMP 190 C. TEMP 200 C. TEMP 250 C. Ex. COMPOSITIONPEEL PEEL PEEL PEEL No. (MOLE %) STRENGTH STRENGTH STRENGTH STRENGTHCURING Tg C-1 BPDA/BTDA/RODA/ NO NO NO NO 70 C.-150 231.5 HMD ADHESIONADHESION ADHESION ADHESION 1 HR 90%/10%/100%/ 150-250 C. 0% 1 HR. 330 C.5 MIN. C-2 BPDA/BTDA/RODA/ NO NO NO NO 70 C.-150 203 HMD ADHESIONADHESION ADHESION ADHESION 1 HR 90%/10%/90%/ 150-250 C. 10% 1 HR. 320 C.5 MIN. C-3 BPDA/BTDA/RODA/ NO NO NO  5.5 PLI SAME 206.9 HMD ADHESIONADHESION ADHESION 90%/10%/75%/ 25% C-4 BPDA/BTDA/RODA/ NO NO NO 10.0 PLI70-150 C. 206.1 HMD ADHESION ADHESION ADHESION 1 HR 90%/10%/65%/ 150-250C. 35% 1 HR 300 C. 5 MIN. C-5 BPDA/BTDA/RODA/ NO NO NO 11.0 PLI 70-150-1HR 199.4 HMD ADHESION ADHESION ADHESION 90%/10%/55%/ 150-250 C. 45% 1 HR320 C. 5 MIN. C-6 BPDA/BTDA/RODA/ NO FILM NO FILM NO FILM NO FILM SAME —HMD 90%/10%/10%/ BRITTLE OFF BRITTLE OFF BRITTLE OFF BRITTLE OFF 90%GLASS GLASS GLASS GLASS C-7 BPDA/BTDA/RODA/ NO FILM NO FILM NO FILM NOFILM SAME — HMD 90%/10%/0%/ BRITTLE OFF BRITTLE OFF BRITTLE OFF BRITTLEOFF 100% GLASS GLASS GLASS GLASS

1-10. (canceled)
 11. A multi-layer polyimide composite comprising afirst layer and a second layer wherein the first layer comprises apolyimide adhesive composition in accordance with claim 2, and whereinthe second layer is a polyimide with a glass transition temperaturegreater than 250° C.
 12. A three-layer polyimide composite comprisingtwo outer layers and one inner layer wherein the two outer layers are apolyimide adhesive composition in accordance with claim 3, and whereinthe inner layer is a polyimide with a glass transition temperaturegreater than 250° C.
 13. A two-layer polyimide composite comprising afirst layer and a second layer wherein the first layer comprises apolyimide adhesive composition in accordance with claim 3 and whereinthe second layer is a polyimide with a glass transition temperaturegreater than 250° C., said two-layer polyimide composite furthercomprising a metal layer bonded to said first layer.
 14. A three-layerpolyimide composite comprising two outer layers and one inner layerwherein the two outer layers are a polyimide adhesive composition inaccordance with claim 2, and wherein the inner layer is a polyimide witha glass transition temperature greater than 250° C., said three-layerpolyimide composite further comprising two metal layers, an upper metallayer and a lower metal layer, wherein the upper metal layer is bondedto one said outer layer, and wherein the lower metal layer is bonded tothe other said outer layer.
 15. (canceled)
 16. A flexible polyimidemetal-clad laminate having a polyimide adhesive layer and a metal layer,wherein the polyimide adhesive comprises a polyimide base polymer, thebase polymer being synthesized by contacting an aromatic dianhydridecomponent with a diamine component, wherein the diamine component is 50to 90 mole % aliphatic diamine and 10 to 50 mole % aromatic diamine, thepolyimide base polymer having a glass transition temperature from about150, 160, 170, 180, or 185 to about 190, 195, 197 or 200° C.
 17. Aflexible polyimide metal-clad laminate according to claim 16, whereinsaid metal layer is formed on said polyimide adhesive film by firstsputtering and then electro-plating a plurality of metal atoms onto asurface of said polyimide adhesive.
 18. A flexible polyimide metal-cladlaminate according to claim 16, wherein said polyimide adhesive layer iscast onto said metal layer in a form of a polyamic acid and then driedand cured to form a polyimide.
 19. A flexible polyimide metal-cladlaminate according to claim 16, wherein a bond strength between thepolyimide layer and the metal layer, as determined by ASTM MethodIPC-TM-650 Method No. 2.4.9.D, is greater than 2, 3, 4, 5, 6, 7, 8, 9,or 10 pounds per linear inch (pli).
 20. The flexible polyimidemetal-clad laminates of claim 16, and wherein the lamination temperatureof the polyimide adhesive film to the metal foil is in the range of from180° C. to 250° C. 21-31. (canceled)